Dihydroxyalkanes such as ethylene glycol and 1,2-propanediol (propylene glycol) have uses in a wide variety of applications, including as monomers in polyester resins, in antifreeze and deicing fluids, in the manufacture of food, drug and cosmetic products, and in liquid detergents.
Propylene glycol is predominantly currently produced by oxygenating propylene to produce the epoxide, propylene oxide. Propylene oxide is then typically reacted with water to form the desired 1,2-propanediol. Because the process begins with propylene, the cost to make propylene glycol has historically been linked to the price of oil and other hydrocarbon non-renewable resources.
Accordingly, in recent years significant resources have been devoted to the development of processes to make biobased propylene glycol from renewable resources, and biobased propylene glycol is now commercially produced by the hydrogenolysis of glycerol. A process for the manufacture of biobased propylene glycol by the hydrogenolysis of glycerol is described, for example, in U.S. Pat. No. 6,841,085 to Werpy et al., wherein compositions containing a 6-carbon sugar, sugar alcohol or glycerol are reacted with hydrogen over a Re-containing catalyst.
Glycerol is produced commercially as a byproduct of the biodiesel process, but requires costly purification to be useful as a feed to such a catalytic process for producing biobased propylene glycol. As well, biodiesel production is to a large degree dependent upon regulatory requirements and governmental incentives, so that carbohydrate-based routes to a biobased propylene glycol have also been proposed.
For example, U.S. Pat. No. 6,479,713 also to Werpy et al. (2002) proposes a process for the synthesis of propylene glycol by reacting hydrogen with a 5-carbon sugar, sugar alcohol, or lactic acid over the same Re-containing multimetallic catalyst. The hydrogenation of lactic acid is described as preferably being conducted in a temperature range of 110 to 200 degrees Celsius, and more preferably in the range of 140 to 170 degrees Celsius, under neutral to acidic conditions. Examples are provided wherein a solution of about 20 weight percent of lactic acid in deionized water is reacted with hydrogen at 2500 psi and at 150 degrees Celsius.
WO 2000/030744 (also published as U.S. Pat. No. 6,403,844 to Zhang et al), U.S. Pat. No. 6,455,742 to Cortright et al. (2002) in addition to the above-mentioned U.S. Pat. No. 6,479,713 to Werpy et al. (2002) that hydroxycarboxylic acids obtainable by fermentation of crude biomass, for example, lactic acid, have for some time also been viewed by the art as promising for glycols production.
More particularly, Simakova reports that a liquid phase hydrogenation of the carboxyl groups in such acids had been recognized as requiring a high hydrogen pressure and high catalyst loadings, so that various patents and articles had proposed the more facile reduction of hydroxycarboxylic acid esters. Still, according to Simakova, the reduction of hydroxycarboxylic acid esters in the liquid phase necessitated the use of high hydrogen pressures, referencing for example the work of Adkins et al. wherein hydrogen pressures from 20 to 30 MPa were used (see, e.g., Adkins and Billica, J. Am. Chem. Soc., vol. 70, pg 3118 (1948)), so that Simakova proposed a gas phase process wherein a mixture of esters of hydroxycarboxylic acid and hydrogen were reacted in the gas phase in the presence of a catalyst containing a “mixture of copper and/or oxide of copper and/or hydroxide of copper and/or salt or mixture of salts of copper and of inorganic acids of the element IVb, Va and VIa groups of periodic system, and oxide or mixture of oxides of the element IVb, Va and VIa groups of periodic system”.
U.S. Pat. No. 6,455,742 to Cortright et al. proposes a process for catalytically reducing the carboxylic acid group of hydroxycarboxylic acids to a hydroxyl group, using hydrogen pressures of less than 50 atmospheres and a zero valent copper catalyst. The catalyst may be supported on silica, and hydroxyl groups on the silica may be capped with hydrophobic groups and silanes, such as trialkylsilanes. A vapor phase process is contemplated in one embodiment, while in other embodiments the lactic acid contacts the catalyst and hydrogen in the presence of water.
The last patent publication referenced by Simakova, namely, U.S. Pat. No. 6,403,844 to Zhang et al., provides a process for the production of propylene glycol in an aqueous reaction mixture of lactic acid and hydrogen with “an essentially pure ruthenium catalyst on an inert support at elevated pressure and temperature”. Zhang et al. indicate that an important aspect of their invention is the capacity to convert unrefined or crude lactic acid in the form of a bacterial fermentate or in the form of lactic acid containing lactate salts (e.g., alkali metal, alkaline earth metal or ammonium salts of lactic acid) to propylene glycol, whereas the conversion processes of the prior art are described as requiring pure preparations of lactic acid or its esters.
Finally, Luo et al., “Effect of promoters on the structures and properties of the RuB/γ-Al2O3 catalyst”, Journal of Molecular Catalysis A: Chemical, vol. 230, pp. 69-77 (2005) is similarly directed, in reviewing the effect of various promoters (Co, Fe, Sn, Zn) on the performance of a monometallic ruthenium/boron on alumina catalyst in the liquid phase hydrogenation of ethyl lactate to propylene glycol.